The present invention relates to a method and apparatus for generating diffraction gratings on a photo-sensitive material on a substrate, and more particularly, the present invention relates to a method and apparatus for generating sinusoidal structures on photo-sensitive material on substrates by the recordation of an interference pattern produced by four plane waves of light incident thereon.
Two dimensional periodic structures such as gratings, echelles and rulings are used in many optical systems. Developments in laser technology have provided dramatic improvement in the understanding and fabrication of these structures. For example, the brightness and coherence of laser light has permitted accurate measurement, control and generation of periodic structures. The ability to expose photo-resist materials with lasers has lead to a holographic method of fabricating such structures. Gratings made by the holographic method are commonly referred to as holographic or interference gratings, and such gratings can be made with exceptional accuracy and clarity when compared to alternative prior art methods such as mechanical ruling and lithography.
As with all methods of grating fabrication, two basic problems occur. First, it is difficult to produce a consistent groove shape. Any variations in the shape of the groove will generate stray light and alter the efficiency. Second, it is difficult to ensure that each groove is in the correct position, or alternatively stated, that the period of the grooves is consistent. Low frequency errors in the period of the grooves will effect the quality of the diffracted wavefront and will cause distortion in the spectral imaging properties of the grating. Middle and high frequency errors in the period will alter the level of light which is found between diffracted orders. Low, middle and high frequency periodic errors in general are responsible for the generation of xe2x80x9cghostsxe2x80x9d in the field and random errors in the period give rise to background scatter known as xe2x80x9cgrassxe2x80x9d.
The current techniques for fabricating accurate gratings are mechanical ruling, lithograph and interference.
The mechanical ruling method is known to create the greatest amount of stray light, grass and ghostings in gratings. Masks generated by Lithographic techniques (locally exposing a photo-sensitive material with a serially scanned, high energy electron beam have limited control over the groove shape. Errors in the fabrication of such gratings by this technique depend on the exposure apparatus. The trajectory of the electron beam is a raster exposure over discrete blocks of an entire grating pattern. Such technique results in xe2x80x9cstitchingxe2x80x9d errors. Gratings generated by the recording of interference patterns on a photo-sensitive material are known to produce the lowest stray light, have superior wavefront quality and be practically free of ghosts and grass. Drawbacks to the recording of interference pattern technique include the limited control over the groove shape, grating period and overall area of exposure.
Two beam interference pattern recordation for the fabrication of gratings is well known. Such a technique comprises exposing a photo-sensitive material to the intensity pattern generated by the interference of two plane waves of light at equal and opposing angles from a normal to the photo-sensitive material surface. Subsequent development of the exposed and unexposed photo-sensitive material yields a surface having a periodic grating structure due to the preferential dissolution of the resist in the presence of the developer. The period of the resulting structure is determined by the equation.   Λ  =      λ          2      ⁢      n      ⁢              xe2x80x83            ⁢      sin      ⁢              xe2x80x83            ⁢      θ      
where xcex9 is the period of the grating, xcex is the wavelength of light, n is the index of refraction of the medium where interference occurs and xcex8 is the angle of the incident light on the surface of the substrate as measured to the normal of the surface. For periods much larger than the wavelength of the light (where xcex9 is much greater than xcex), very small angles of incidence are necessary. The small angular displacements required for relatively large periods severely limit the size of a region of a substrate which can be exposed to the interference pattern. For example, the generation of interference patterns having periods separated by 20 microns with light of a 0.488 xcexcm wavelength require the incident plane waves of light to be 1.45 degrees from the normal drawn from the surface. Slight variations in the incident angle will cause dramatic changes in the period. Variations in the incident angle on the local scale are equivalent to wavefront distortions. For accurate generation of ronchi rulings using two beam interference, there are excessive burdens on the engineering of exposure optics. The sensitivity of variations in the incident angle makes the two-beam approach difficult to implement.
The method and apparatus of the present invention utilizes a four plane wave holographic approach which allows for much larger exposure areas on a photo-sensitive substrate. The method and apparatus of the present invention also allows creation of a period by choosing the angles of incidence for the various plane waves.
The present invention relates to a method and apparatus for fabricating diffraction gratings on a substrate having a photo-sensitive material thereon. The method comprises the step of simultaneously exposing the photo-sensitive material on the substrate to an interference pattern produced by a first pair and a second pair of plane waves. Each plane wave of the first pair is incident on the substrate at an equal but opposing angle xcex8, xe2x88x92xcex8 of incidence subtending from a line normal to the substrate surface. Each plane wave of the second pair is incident on the substrate at an equal but opposing angle xcfx86, xe2x88x92xcfx86 of incidence also subtending from a line normal to the substrate surface. The pattern of interference may be controlled by changing the angles of incidence of the first pair of plane waves and the angles of incidence of the second pair of plane waves. After exposure, the photo-sensitive material is developed so as to remove material exposed to incident light.
An object of the present invention is to provide a method and apparatus for fabricating gratings on a substrate having a photo-sensitive material thereon.
Another object of the present invention is to provide a four plane wave holographic method and apparatus for exposing substrates having photo-sensitive materials thereon to larger areas of interference patterns.